What is the difference between "to be right," "was right," and "is right" used in this sentence? And why using "was right" and "is right" possible? Is it because it is used the same way as "doctrine which I think was right"?

Also, is it possible to change this question

What would you predict the mass of rocket to be?

to this

what would you predict the mass of rocket is?

since the answer would usually be "I predict that the mass of rocket is...", not this: "I predict the mass of rocket to be." Is it possible? Thank you!

I looked at it, but that was not what I am asking. I am confused because I believe part comes before the doctrine to modify it with many different things as " was", "is" and "to be". I know it is ok to use all of them, but I don't know why.
– scarecrowAug 18 '15 at 0:28

Why would there need to be a justification for why all of those are just fine? By the way, you forgot the fourth choice of leaving out the final verb altogether: It is the doctrine I believed right.
– tchrist♦Aug 18 '15 at 0:30

Because sometimes I get confused as to what to use. Like in this question: What would you predict the mass of rocket to be (is)?
– scarecrowAug 18 '15 at 0:34

Merely use any one of those you please. There isn’t a right and wrong here.
– tchrist♦Aug 18 '15 at 0:37

1 Answer
1

All three of these sentences are grammatical, and they all mean the same thing.
Further, all three lack the same thing -- a relative pronoun marking a relative clause:

It is the doctrine that/which I believed to be right.

It is the doctrine that/which I believed was right.

It is the doctrine that/which I believe is right.

All three of these sentences are grammatical and synonymous as well.

Notice also that in all three relative clauses, the relative pronoun, whichever it is,
is extracted from a clause below the believe clause; so let's reconstruct that originals:

I believed the doctrine to be right

I believed (that) the doctrine was right

I believed (that) the doctrine is right

Once again, all grammatical and all synonymous. The relative clause formation rule extracts the doctrine and then deletes the relative pronoun because it's not the subject of its relative clause
(I is already the subject).

This leaves a gap between I believed and its object clause, which is reduced to a verb phrase only.
So why are there three variants? Because believe can take an infinitive complement

Max believed Freddie to be guilty of the murder.I believed the doctrine to be right.

as well as a tensed that-clause, of either tense:

Max believed (that) Freddie was guilty of the murder.I believed (that) the doctrine was right

Max believed (that) Freddie is guilty of the murder.I believed (that) the doctrine is right

and here all the clause-marking that's have been deleted.
They can't occur with an infinitive, only with a tensed clause.